The importance of room acoustics is often overlooked, even by those
who consider themselves serious listeners. People who obsess over vanishingly small
amounts of distortion or frequency response errors in their gear accept response
deviations of 30 dB or more added by their room. Often they have no idea how bad
their room really is! This graph shows the low frequency response measured in a typical
small living room:

Figure 1: This is the low frequency response you can
expect in a room without any bass traps.

But a skewed frequency response is only one of the
problems caused by poor room acoustics. Rooms also resonate and ring, sustaining some
frequencies longer than others. If you clap your hands in an empty bedroom you can hear
the pitched "boing" sound caused by reflections bouncing repeatedly between
opposing surfaces. The pitch of the boing is related to the distance between the
boundaries. The same thing happens at bass frequencies, but hand claps don't have enough
low frequency content to excite the resonances. The resonances are still there, and they
damage bass clarity, but you need an appropriate test signal to measure them.

Even if the steady state room response were perfectly flat, if 1 KHz
sustains for half a second longer compared to other frequencies, the perceived volume at
that frequency will be higher due to the additional energy over time. So one important
metric for room measurement is decay time versus frequency. The goal is for decay times to
be more or less uniform over as wide a range as possible. In larger rooms, excess reverb
and ambience cloud detail, and in smaller rooms "early" reflections - those
arriving at your ears within about 20 milliseconds of the direct sound - have a similar
effect. Besides obscuring detail, early reflections also create a particular type of
response error called comb filtering. Indeed, all
acoustic problems are caused by reflections off the walls, floor, and ceiling.

I separate room acoustics - both measurements and treatment - into
two frequency ranges: bass below about 300 or 400 Hz and mid/high frequencies above 400
Hz. For low frequencies it's important to see as much detail as possible. This means
measuring and displaying the response at high resolution in order to see the true extent
of peaks and nulls. This next graph shows the exact same measurement at two different
resolutions:

Figure 2: Standard 1/3 octave averaging hides a lot
of detail, as you can see when the same data is displayed at 1/12 octave resolution.

At mid and high frequencies it's more appropriate to
use averaging. Small changes in microphone placement have a huge effect on the measured
response. A graph of the high frequency response that is not averaged is riddled with so
many peaks and nulls it's difficult to see the forest for the trees, so to speak.

Why we measure

Many people believe that room measuring is needed to know how to
approach treating a room, to determine the number and placement of bass traps, mid/high
frequency absorbers, and diffusors. But in most cases you can treat a room effectively
without measuring at all. The basic goal is to put bass traps in the room corners,
mid/high frequency absorbers at the side-wall and ceiling reflection points, and
optionally absorbers and/or diffusors on the rear wall behind the listener. Treating the
rear wall is more necessary when that wall is closer than ten feet behind your head.

Originally, I got into room measuring mainly to show people how
terrible typical rooms are. As I said earlier, most people have no idea how badly their
room damages sound quality. They worry whether their loudspeakers are flat, while ignoring
how much worse the response is made by their room. Of course, measuring is useful to
assess the improvement after adding treatment, and to compare the benefit from different
acoustic panel placements. It's also useful to see if more treatment is still needed, and
at what frequencies the problems remain.

What we measure

The two main things we measure are raw frequency response as shown
in Figure 1 above, modal ringing and reverb, and impulse response. Ringing is similar to
reverb except it sustains some frequencies more than others. If you sing different notes
in a large space like a church or auditorium, all of the notes will decay at more or less
the same rate. In small rooms the decay times become much more frequency-dependant,
especially at low frequencies. To view ringing decay time we use a "waterfall"
graph like this:

Figure 3: The graph above is derived from the same
data shown in Figure 1, but this graph also shows modal ringing and the decay times.

In this type of graph the "mountains" come
forward over time, and represent resonating peaks in the response. When some bass notes
played in a room sustain longer than others, they will seem louder, and indistinct,
creating the effect known as "one-note bass."

The last type of graph shows the impulse response, which lets you
see the timing and strength of individual reflections. We'll get to that later in this
series, along with graphs that show reverb decay times versus frequency.

In this introduction I explained the errors introduced by all
untreated rooms. Once you know the audible problems a room causes, you then know what
needs to be measured. And once you know what is being measured, you can better understand
what the graphs displayed by room measuring software mean. Top

In this piece we will outline
the hardware you will need. Bear in mind that there are many other options that would work
just as well as the ones that I have recommended here. When you are considering other
products, be sure to check if they have the basic functionality that is required. Beyond
that you can't really go wrong!

You don't need to spend megabucks on a measurement microphone for
room acoustics. At a minimum you should look for a microphone that has been measured to
determine how that microphone sample varies from the target flat frequency response. A
microphone that has been measured like this is called a calibrated microphone. The
microphone will be supplied with a calibration file or sufficient information to allow you
to create a calibration file. This file is used to compensate for the frequency response
of the microphone in the measurement software. Unfortunately most cheap (but very
suitable) microphones don't come with a calibration file, therefore you either need to
spend more for one that does, or have your microphone calibrated. Note that a generic
calibration files for a particular brand and model are normally not useful, since there
are manufacturing vagaries which means each microphone sample will measure differently.

EW Note: In my experience, microphones don't need to
be calibrated unless you're a professional acoustician who's being paid to provide highly
accurate readings. The RealTraps article Comparison of Ten Measuring Microphones
shows the test results for many small diaphragm microphones commonly used for room
measuring. As you'll see there, many of the budget models have a response very similar to
the most expensive brands. However, if you want to know for sure that your measurements
are accurate, having your microphone calibrated makes sense. And it's not prohibitively
expensive.

Recommended option:

Behringer ECM8000 microphone, optionally calibrated byCross Spectrum Labs or another
reputable calibration house.NOTE: If you mention RealTraps when having your microphonecalibrated, Cross Spectrum Labs will give you a $5 discount.

2) Computer soundcard

A computer soundcard for use in room acoustics measurement must be
what is called full duplex - that is, it can play and record at the same time. For simple
measurements you really only need one input and one output, although some of the
measurement packages use the second channel in a two-channel soundcard to perform what is
called a loop-back measurement. This can be used to further improve the accuracy of an
acoustic measurement (more on this later). The other things that are required are a
microphone level input and phantom power. Microphones output a very low voltage and so
need a specialized high gain amplifier to bring them up to line level. This is typically
provided by a dedicated "mic input," most of the time with XLR plug connection,
although some soundcards use a TRS input. Most quality condenser microphones also require phantom
power, which is typically 48 volts DC. Without this power the microphone will not
work. Because of need for a preamp and phantom power supply, it is typical to use an
external soundcard.

Recommended options:

M Audio MobilePre USB

E-MU Tracker Pre USB 2.0

EW Note: You'll often see the SoundBlaster MP3
USB sound card suggested because it works well enough for room measuring and is
inexpensive. But this sound card does not include phantom power, so it's useful mainly
when you'll use an SPL meter as a microphone. This SoundBlaster card doesn't have its own
external power supply either, instead relying on the computer's USB port for its own
power. The USB standard requires each port to be able to deliver 500 milliamps to
connected devices. But some laptops can't provide that much current. When I bought a
SoundBlaster USB sound card a few years ago to use with my Dell Inspiron laptop, the sound
card didn't work. I thought it was defective so I returned it and got another. Same
result. So I called Dell tech support, and learned that my laptop can output only 300
milliamps from each USB port! So I bought a Presonus FireBOX, which has an external
"wall-wart" power supply, and that has worked perfectly ever since.

Added January 7, 2013: The Dayton Audio UMM-6 USB Microphone can
be used with any Windows or Mac computer and doesn't require a separate preamp.

3) Microphone stand

A stand is essential to place the microphone at the various
locations typically used for room acoustics measurement (e.g. at ear location). At a
minimum the stand should be stable, and adjustable in both the vertical and horizontal
planes. Adjustability in the horizontal plane is provided by what is called a
"boom" arm. Any generic stand should be fine; don't pay more than $30 or $40!

4) Cabling

The cabling needed depends on which hardware you buy. If you follow
my recommendations above you will need:

One XLR to XLR microphone cable. I would get a cable at least 20'
long to allow easy movement. There's nothing worse than having a microphone cable that's
too short!

Two 1/4-inch (single ended rather than balanced) to RCA plugs, at
least 6' long. If your amplifier has XLR inputs only then you will need to get TRS to XLR
cables. Top

Over the years I've
owned several popular room measuring programs, but these days I use mostly Room EQ Wizard. This program
is offered as freeware, and it works beautifully with Windows. It claims to also work with
Linux and Mac OSX 10.4 or higher, though I've never tested either of those. This article
explains how to configure REW. In the next part Nyal Mellor will offer similar information
about FuzzMeasure, a popular and highly capable program for Mac computers.

As with most room measuring software, REW uses a sine wave sweep as
its signal source. There are many advantages to using a sine wave sweep versus pink or
white noise. The main advantage is a sweep offers a higher signal to noise ratio. When the
software analyzes the sweep as recorded through your microphone, it can apply a tracking
filter to the recorded tones. This is a sweepable filter that is applied internally by
the software as it analyzes the recorded sweep. The filter passes only the frequency of
interest at that moment, thus filtering out other sounds such as loudspeaker distortion,
preamp hiss, your own breathing, and footsteps or outdoor traffic and barking dogs. A sine
sweep also takes the software less time to do a measurement, especially at low
frequencies. When noise is used as a signal source, the noise has to play for a longer
time and be averaged. Indeed, one huge advantage of dedicated room testing software
- versus an old fashioned Real Time Analyzer with pink noise - is you can measure the room
once, then display many different types of data and graphs later.

Configuring REW

The first step, after installing REW, is configuring it to work with
your computer's sound card. Figure 4 shows the setup screen, accessed from the large
Settings button near the top left of the main screen. I'll address only the choices needed
to prepare REW for normal use. Anything not addressed in this article can be left at the
default setting.

Figure 4: The REW setup screen lets you set the sound
card's input and output, sample rate, and other parameters.

I generally use a sample rate of 44.1 KHz simply
because that's what I use for recording music, though 48 KHz is fine too. Most computers
have only one sound card, but some have internal and external cards, and some cards have
multiple inputs and outputs. Figure 5 shows the drop-down box where you select which sound
card to use.

Figure 5: This portion of the setup screen above is
where you select which sound card(s) to use.

As you can see, my studio computer has two physical
sound cards - a SoundBlaster X-Fi and a multi-port Delta 66 made by M-Audio. I use
channels 1 and 2 on the Delta 66 for REW, but I could just as easily use channels 3 and 4.
A similar drop-down selector lets you pick the input sound card, which could be different
than the output. Either the left or right channels can be used for the microphone input.
In this case I use the left channel. The output sweep tone is sent to both channels at the
same time, so there's no selection for that.

You also need to tell REW which speakers you plan to use for setting the playback and
record levels - the main speakers or subwoofer. This selection is shown in Figure 6,
though you'll read below why I don't use this to set levels.

Figure 6: Here's where you tell REW that you'll use
the main left and right speakers for level checking.

Using REW

When measuring the low frequency response in a room, it's important
to measure using both the left and right speakers sounding at once, plus the subwoofer. If
you measure using only the subwoofer, data within an octave or so of the crossover
frequency is not accurate. I'll have more to say about measuring with one versus all
speakers playing in a moment.

You can now close the Setup screen, and continue on to setting the
output and input levels levels, and do a test sweep. Click the Measure button in the upper
left of the main REW screen, and you'll see the Measurement screen shown in Figure 7
below.

Figure 7: This screen is where you'll run the actual
measurement sweeps.

In most cases you should set the lower and upper
sweep range limits to 20 Hz and 20 KHz respectively. If your sub goes below 20 Hz you can
use a lower frequency. If for some reason you feel the need to measure the response to
higher than the 22,050 Hz limit of a 44.1 KHz sample rate, you can go back to the setup
screen and select 48 KHz. Then you can measure up to 24 KHz. The only time I limit the
sweep range is if I'm doing many measurements in a row and don't want to wait for the full
sweep every time. So if you're using REW to help find the best location for a subwoofer,
you could set the upper limit of the sweep to 200 Hz. Again, even if your sub crosses over
at, say 100 Hz, it's still active to an octave or so higher, depending on the crossover
slope (dB per octave).

For some reason the Check Level button plays pink noise instead of a
sine sweep, and the level is different than the sweep tone. So I just run a sweep to set
levels, then cancel the measurement. The playback meter should read around -12, and you'll
adjust your receiver's volume control so the sweep sounds fairly loud in the room. But not
so loud it sounds like you'll damage your speakers, of course! Common sense applies here.
The only reason you need to play the sweep fairly loud is to drown out all ambient noise
in the room by at least 30 or 40 dB. Nulls 30 or more dB deep are common in most rooms,
especially with little or no acoustic treatment. So if the ambient room noise is only 20
dB softer than the sweep at a null frequency, the true null depth will be hidden. Of
course, the tracking filter built into REW mentioned earlier helps.

Most room measurements are done with the microphone at the listening
position, at ear height, and pointing straight up to not favor either speaker. It can be
useful to measure at other locations, such as very close to the speaker to measure its own
response at mid and high frequencies with less influence from the room. But for the most
part, what matters is the response you hear at your usual listening seat. So place your
microphone or SPL meter on a stand, with no obstructions on either side or above or below
- including yourself!

Next, click Start Measuring to display the play and record level
meters (not shown), then adjust your microphone preamp's gain so the record meter averages
around -20 or so, but peaking safely below 0. If the record level is too low, or too high,
REW will tell you. If you don't get an error message, the levels are fine and you can do
another sweep "for real" now.

Earlier I mentioned playing the sweep tone through all speakers at
once. With most pop music, and a lot of other music, and all LP records, the bass is
centered left and right sounding equally through both speakers. So to know the true
response at low frequencies for music you listen to, you should mimic that and play the
sweep through both the left and right speakers too. If you use a subwoofer, that should
also be engaged for the same reason. However, it's useful to also test each speaker
separately. This will quickly reveal unusual problems such as a blown midrange driver, or
a severe peak or null caused by a reflection on one side only.

Using the RTA function

Finally, I'll share a clever trick that can help you to place
speakers, subwoofers, and even bass traps more efficiently. It takes REW only ten seconds
to do a sweep and show the results, but it's still tedious to move a speaker or bass trap
a few inches, measure, display, and repeat. Going back and forth constantly between
handling the speaker and working your computer gets old very fast. And you have to keep
deleting all the intermediate measurements. So to speed up the process I created a small
(180 kb) Wave file that contains this Sweep Tone ranging from 20 to
400 Hz.

Download this file to your computer, then set it to run continuously
in Windows Media Player, or any other program that "plays nice" with Windows
sound drivers and shares your sound card with other programs. Then switch to REW's
Spectrum display (tab) as shown in Figure 8 to view the Real Time Analyzer (RTA).

Figure 8: In the REW Spectrum tab, the red Record
button at the lower right puts REW into its RTA mode and updates the screen continuously.

Set all of the RTA parameters as shown in Figure 8 to
get the highest resolution, then click the red Record button to enable the RTA display.
Next, start the sweep playing in your media player program with the volume fairly loud in
the room. Adjust the microphone preamp level until you can see the frequency response
graph line on the screen. (You may also need to adjust the Graph Limits dB range to see
the entire response line on the screen.) Now you can experiment with speaker placement, or
microphone placement, or bass trap locations, and see the result of your changes
immediately. Pretty cool! Top

For those
of us with Apple Mac computers running OSX there are few choices for room measurement
software. FuzzMeasure is one ($150); another is
SMAART ($895), which is much more
expensive and mainly targeted at the professional live sound reinforcement community.

Note that FuzzMeasure requires OSX version 10.5 or higher. It will
not work with 10.4. The rest of this article assumes that you have already installed /
started up FuzzMeasure, installed the drivers for / plugged in your soundcard, and set up
your measurement rig as per the instructions above.

Configuring FuzzMeasure

You will need to modify the Audio Capture Settings to get
FuzzMeasure to work with your soundcard. These are accessible by clicking on the Capture
Settings icon in the top bar in FuzzMeasure.

On the Playback Settings select your soundcard by name

On the Record Settings select your soundcard by name. You will also
need to select the channel that FuzzMeasure will record on.

Use of the Automatic Correction is optional in my opinion.
Auto-correction basically subtracts any frequency response anomalies that your soundcard
has from the measurement taken. Most good quality soundcards will have a very flat
frequency response in the frequency range of interest (e.g. I think the M Audio MobilePre
is 0.5 dB down at 20Hz), and so do not need correction. If you desire, you can use the
Automatic Correction feature by setting up the input and output channels. You then simply
need to connect a suitable cable between the input and the output. If you are using the M
Audio unit then this is a 0.2m 1/4" unbalanced connector, commonly sold as a
"patch" cable.

Next, set up FuzzMeasure so it averages a couple of
sweeps, rather than relying on one measurement. This can be set through the top menu,
under Measurement then Averaging.

Configuring your sound card

The second thing you should do is set up any software volume
controls that your soundcard has to ensure that the input and output levels are
appropriate and that monitoring is disabled. In all cases you should set input volume
level to maximum and monitoring to off. If you are connecting your soundcard to your
system through an amplifer with a volume control, then you should set input volume to
maximum. If you are connecting your computer directly to an amplifier that does not have a
volume control (e.g. in my case I connect straight to a power amp as I do not have a
pre-amp in my system) then you should set the output volumes very low.

Setting Levels

I am assuming that you do not have access to a microphone calibrator
to set absolute SPL levels. A microphone calibrator is a small device that fits over the
end of the microphone and plays a 1 KHz sine wave at 94 dB. Most of us don't own a
calibrator, so don't worry.

When you are ready (with the volume turned down low on your
amplifier!), set the input level dial on your soundcard to around the midway point. Now
hit the Measure button in the top bar in FuzzMeasure.

You will get a warning message, which you should read (!) and then
press Play.

You should hear a sine sweep. Now turn up the volume
so the sweep sounds fairly loud in the room. But not so loud it sounds like you'll damage
your speakers, of course! Again, common sense applies. The only reason you need to play
the sweep fairly loud is to drown out all ambient noise in the room by at least 30 or 40
dB.

Once the measurement has completed (remember there will be three
sweeps since we are using averaging), the frequency response will be displayed in the main
window. A good way to check if the level is set right is to look at the highest point on
the chart. It should be around -12dB or above.

Getting the measurements

Once you have a good measurement showing in the main window you can
interpret it in many different ways.

Low resolution, full range frequency response
Set averaging to 1/3 octave under the Frequency > Smoothing in the top menu.

High resolution, bass
range frequency response
Set averaging to 1/24 octave under theFrequency > Smoothing in the top menu, and
Frequency Range through the horizontal bar on the right side of the main measurement
window to end at 300Hz. I turned off my EQ for these measurements, that's why they look
ugly!

Reverberation time
You can access this using one of the plugins. Select under Plugins > Reverberation Time
in the top menu.

Cumulative spectral decay
This is used to understand low frequency decay in a room and see the effects of resonances
from room modes) - select Plugins > Waterfall in the top menu. You will also need to
set some of the options for this to display meaningful data. In particular set the Max
Frequency to 300Hz , Min Magnitude to -35dB, Smoothing to 1/24 Octave and Resolution to
High.

Envelope Time Curve
This is used to understand reflection strength and distance - access this through Impulse
> Display Type > Envelope Time Curve in the top menu.

In this final
installment of the Room Measuring series I'll show the most important graph types
displayed by room measuring software. For these examples I'll use a set of Before / After
measurements taken in a typical "extra room" size space 16 by 11.5 by 8 feet,
with and without extensive acoustic treatment. This is the same data I measured for the Hearing is
Believing video on my company's web site. The software used is Room EQ Wizard.

Frequency Response

Everyone is familiar with frequency response graphs, as shown below
in Figure 9.

Figure 9: This graph shows two measurements made in
the same room with and without extensive bass traps and other acoustic treatment. I'm sure
I don't have to explain which colored lines are before versus after!

In this graph the display is limited to the range
below 400 Hz, since that's where bass traps have the most profound effect. Frequency
response problems at mid and high frequencies are easy to tame with relatively thin
absorbers. But bass problems are much more difficult to tackle. Graphs like this help you
assess the improvement after adding bass traps and changing their position, and changing
the placement of loudspeakers, including subwoofers, and even moving the listening seat.
The speakers used for this test are flat to just below 40 Hz.

Note that the graph above is at the highest resolution REW offers.
REW also lets you apply averaging at various resolutions, such as 1/3 octave. But that is
not appropriate at low frequencies because it hides the true extent of peaks and nulls.
However, averaging is useful at mid and high frequencies. The response measured at higher
frequencies is often riddled with peaks and deep nulls due to comb filtering. Comb
filtering occurs in untreated rooms due to reflections from the side walls, floor, and
ceiling. But comb filtering exists even in well treated rooms due to small differences in
arrival time from the left and right speakers. If the measuring microphone is not precisely
centered, equidistant to both speakers, that alone can cause a series of many peaks and
deep nulls. This is shown in Figure 10 below.

Figure 10: This graph shows the same measurement in
Figure 9 above, but for the full range out to 20 KHz.

After applying 1/3 octave averaging we get the graph
in Figure 11 below, which is much easier to read and see the overall response without
distraction.

Figure 11: This graph shows the same data as Figure
10, but with 1/3 octave averaging applied to be easier to interpret. (The dip at 10 KHz is
a "measuring artifact" caused by the microphone being too far off-axis from the
tweeters.)

Waterfall Plots

Another very important graph type is called the waterfall plot. This
is much more useful for assessing low frequency problems than seeing just the raw response
because it also shows how some frequencies linger, taking longer to decay. This phenomenon
is called modal ringing, and is caused by resonances within the room itself. The
frequencies that peak and ring longer than others are related to the room's dimensions.
The longer the dimension, the lower the frequency. In this type of graph the
"mountains" come forward over time. I generally set waterfall plots in REW to
show a 30 to 40 dB span vertically, with the Time Range set long enough to see decays over
the first half second or so.

It's also important to set the Window time to at least 200
milliseconds to see enough frequency detail. This setting is in the lower right corner of
REW's Waterfall tab shown at left. The Window time for waterfall plots is similar to the
fractional octave (1/3, 1/12, etc) smoothing used for frequency response graphs. As you
set the Window time longer, the Hz resolution becomes finer showing more detail. I
generally use 300 milliseconds.

The graph below in Figure 12 shows the room when empty, with no bass
traps or other treatment.

Figure 12: This graph is based on the same data as
the others, but shows the low frequency response and the room's modal decay times.
In this type of graph the "mountains" come forward over time.

After adding extensive bass trapping to the room, you
can see in Figure 13 below that the response is much flatter and the decay times are also
much more uniform. Extended decay times at some low frequencies, but not others, is just
as damaging to audio quality as a skewed response. This ringing is the primary cause of
the problem known as "one-note bass," where all notes played by the bass
instrument seem to be at the same pitch even though they're not.

Figure 13: After adding extensive bass trapping to
the room, the response is made flatter and the decay times are also reduced. The goal in
domestic sized listening rooms is to have similar decay times at all frequencies.

Reverb Time (RT60)

The final graph type we'll consider is RT60, shown below in Figure
14. RT60 is acoustician shorthand for "Reverb Time to decay by 60 dB." Most
people consider reverb as taking some amount of time to decay, which is true. But for room
measuring it's useful to see the decay time at each individual frequency band. For
example, a room that has too much thin absorption will decay quickly at high frequencies,
while lower frequencies linger for much longer. This gives an unbalanced sound, similar to
having a high frequency response roll-off. That is, if high frequencies don't linger as
long as lower frequencies, the overall energy in the room is lower even though the
absolute levels coming from the speaker are the same.

Figure 14: This graph shows the Before and After
broadband RT60 decay times. This graph gives data that's similar to a waterfall plot, but
it averages the decay times into octave bands. REW also lets you specify 1/3 octave
averaging for better resolution.

As you can see in the graph above, the decay times
are much faster and much more uniform versus frequency after adding bass traps (and
in this case diffusors) to the room.

Envelope Time Curve (NM)

The measurement below shows sound pressure level on the vertical
axis and time on the horizontal axis. The graph is sometimes normalized so that the direct
sound received from the loudspeaker is set to 0 decibels (dB) and 0 millisecond (ms). This
chart, taken from FuzzMeasure, has not been normalized.

From this graph you can see the level and time of reflections from
room boundaries and other objects within the listening room. The target for this
measurement is typically that the level of reflections should be at least 10 to 15 dB
softer than the direct sound.

Let's pretend there is a strong reflection at 10ms after the direct
sound - the left-most peak at 8 ms on the chart below. To find where this is coming from
in a room you will need to convert time into distance. The speed of sound is 1,125 feet
per second (i.e. the sound from your speakers travels 1,125ft in one second). That equates
to approximately 1.1 feet per thousandth of a second (i.e. 1.1 feet per ms). So in 10 ms
the sound has traveled 11ft. You should then look around your room to identify the
boundaries or objects that are causing the reflection. Remember that the sound has to
travel to and from a boundary before it reaches the microphone position.

Once you find the boundary in question then you can determine if
that reflection is problematic and should be treated. Some reflections are not necessarily
problematic (e.g. side walls away from reflection points), and there are different options
on how to treat the reflection (it can be absorbed or diffused or reflected away).